As is known, currently, for reproducing a graphic representation on a plastic object, for example on an object made of a polyolefin polymer, such as polyethylene or polypropylene, a film is printed with a known printing system (rotogravure or flexographic printing), the film being of, for example, a polyolefin polymer which after the printing operation is manually introduced into a mold.
Subsequently plastic (e.g. polyethylene or polypropylene) is injected into the mold for producing the desired object.
Drawings, signs and the like are currently printed by this method on small objects commonly in use.
This method has a considerable disadvantage in that the film perfectly integrates in the object on which it is applied only in the regions which are not printed, while on the printed film surface, the film only rests on the object surface because the printing inks that constitute a barrier to the molecular penetration between the film and the material constituting the object.
This is the direct consequence of the fact that both polyethylene and the polypropylene, in order to be printed, require a flame surface treatment or high frequency electric discharge (corona effect) surface treatment, before printing with ink. For improving the adhesion of the inks it is also possible to use a hot chemical etching with a highly oxidizing solution, such as chromic acid, oleum, and chlorosulphonic acid.
As has been indicated, the ink creates a "barrier" so that printed portions of both polyethylene and the polypropylene adhere to the material which is cast against the film.
This inconvenience causes the graphic representation to detach especially when the printed portion of the film is subjected to an abrasive action or denting.
Furthermore, the described method only allows printing of small graphic designs on small objects.
In fact, the greater the size of the design, the greater are the chances of its detachment from the object.
The larger is the design, the more difficult it is to arrange it inside the mold, especially if this operation is performed mechanically.
In order to obviate the above drawbacks another type of transfer is used for transferring a design on a plastic object.
To this purpose, by means of a calender for thermoprinting, the colors are transferred from paper to the surface of the object to be printed.
This system also allows printing of a design only on small objects and can be used only with substantially planar surfaces. It is also so expensive that its use is limited to selected and limited cases.